The present invention relates to a magnetron sputtering apparatus and a magnetron sputtering method in which a plurality of ring-shaped flat targets with different diameters are disposed about the same center axis.
A magnetron sputtering technique has been used as a technique for depositing a thin film on a substrate. The magnetron sputtering technique, which enables low-temperature fast sputtering, has become the mainstream of film forming apparatus using the sputtering technique. In the magnetron sputtering technique, plasma is generated around the target through discharge or the like, the resulting plasma ions are thrown into collisions with the target, whereby particles are sputtered, and the sputtered particles are deposited onto the substrate. Thus, a thin film is fabricated.
A magnetron sputtering apparatus of a prior art example is described with reference to FIGS. 8 to 10. FIG. 8 shows the construction of the cathode part of a conventionally used magnetron sputtering apparatus using ring-shaped flat targets. Designated by reference numeral 41 is a center axis, and the cathode part is rotationally symmetrical with respect to the center axis. Numeral 42 denotes a ring-shaped flat target, 43 denotes a magnet placed on the rear side of the targets, and 44 denotes a magnetic field formed by the magnets 43.
The cathode part of such a construction is so arranged that the substrate and the target surface are opposed to each other within a vacuum treatment chamber. After the introduction of sputter gas, the target 42 is fed with power from a high-voltage power supply for glow discharge. Then, a sputtering-use high-density plasma confined by lines of magnetic force is generated. Ions within the plasma colliding with the surface of the target 42, atoms of the target 42 are sputtered so as to be deposited onto the opposing surface of the substrate. Thus, a thin film is formed. The plasma in this process is higher in density at a region 45. The target 42 is eroded by this plasma. The target 42 is eroded at a higher rate around the region 45 of high plasma density, and in particular, the target 42 is eroded at a locally higher rate in the vicinity of an end portion 46 of the region 45, an end as viewed in the direction of lines of magnetic force.
FIG. 9 shows an erosion configuration when the target 42 is exploited up to its use limit by a cross section including the center axis 41. The symmetrical part with respect to the center axis 41 is omitted. Referring to FIG. 9, reference numeral 47 denotes the configuration of the target 42 before sputtering, 48 denotes the sputtered region, and 49 denotes the region remaining unsputtered. As seen in FIG. 9, the vicinity of point F is intensely eroded, causing a V-shaped eroded surface. With the occurrence of an erosion of such a shape, the volumetric use efficiency of the target 42, or the ratio of the sputtered volume to the unsputtered volume of the target when the target is exploited to its use limit, would be about 20%. What is more, only about 10% of the 20% volume would lend itself to deposition onto the substrate. This causes an issue in that the expensive target 42 is not sufficiently exploited. Further, a technical issue is that erosion of this configuration would lead to a variation with time in the film forming rate and the film thickness uniformity.
To solve these issues, there have been proposed techniques as described in Japanese Laid-Open Patent Publications Nos. 5-209266 and 5-179440. The magnetron sputtering cathode as described in Japanese Laid-Open Patent Publication No. 5-209266 is so arranged that magnets are placed on the rear side of the targets, as in the prior art example of FIG. 8, and that ferromagnetic materials are placed on outer and inner circumferences of the targets, whereby magnetic flux is extended beyond the target surface. This arrangement makes the plasma density more uniform and the target erosion also more uniform. FIG. 10 shows the result of erosion when the target is exploited to its use limit with the above arrangement. As in FIG. 9, reference numeral 47 denotes the target configuration before sputtering, 48 denotes the sputtered region, and 49 denotes the region remaining unsputtered. According to FIG. 10, it can be seen that the erosion has progressed more uniformly than in FIG. 9. However, the erosion rate is higher around point G. Further, whereas the volumetric use efficiency of the target in this case is about 40%, only about 10% of the percentage volume would be deposited onto the substrate.
By the conventional method in which the magnet 43 is placed on the rear side of the target 42, a V-shaped eroded surface is formed as shown in FIG. 9. As a result, the target 42 would become locally thinner, and the point at which the thickness of the target 42 reaches below a specified value would be the use limit for the target 42, where the target 42 could not be exploited any more. However, the target 42 still has enough material present therein to form a thin film, posing an issue in that the expensive target 42 cannot be sufficiently exploited. Further, an erosion progress with a locally faster rate would cause the distribution of sputter particles to vary with time, leading to an issue in that the film thickness of the film formed on the substrate would be non-uniform at the final stage of erosion even if the film thickness at the early stage of erosion is uniform. Furthermore, to obtain film thickness uniformity, it is necessary to keep a distance of 70 mm or more between the substrate and target. This would lead to yet another issue that only about 10% of particles sputtered from the target would be deposited onto the substrate.
Also, even in the technique as described in Japanese Laid-Open Patent Publication No. 5-209266, a local erosion would occur around G point as shown in FIG. 10. This accounts for an issue is that the target use efficiency is so low, and the probability of deposition of particles onto the substrate is so low, that variations with time in the film thickness uniformity, film forming rate, and the like would result.
The variation with time in the film thickness in the prior art example is now explained with reference to FIG. 11. FIG. 11 is a chart showing variations with time in the thickness distribution of a thin film formed on a substrate with inner diameter 40 mm and outer diameter 120 mm, by the medium of integrating watts. The vertical axis represents the relative film thickness with the film thickness of the inner circumferential edge of the substrate assumed to be 1, while the horizontal axis represents the distance from the center point. As seen in this chart, the prior art example has an issue is that the distribution of film thickness would undergo a considerable variation with time.